1. Technical Field
This invention relates generally to a diffractive optical element in an image projection system to reduce speckle, and more particularly to a diffractive optical element having a periodically repeating phase mask that is positioned along an intermediate image plane of an image projection system to reduce speckle.
2. Background Art
Modern projection systems, such as laser-based projection systems, facilitate the production of brilliant images created with vibrant colors. The image quality associated with both laser-based and other types of projection systems is continually being improved with advances in technology. Projection systems are becoming smaller and more compact, with some systems being small enough to fit easily into a portable electronic device such as a mobile telephone. Additionally, the power required to produce brilliant images is continually decreasing. Projection systems are becoming less expensive to manufacture as well.
One practical drawback associated with some projection systems is an image artifact known as “speckle.” Speckle occurs when a coherent light source is projected onto a randomly diffusing surface. As lasers generate coherent light, speckle can be prevalent in laser-based systems. However, the problem is not limited to scanned-laser-based systems, as it can also appear in conjunction with laser illumination used in digital light projection systems and liquid crystal on silicon systems as well.
When a coherent light source is used, components of the light combine with other components when the light reflects off a rough surface. This combining works to form patches of higher intensity light and lower intensity light due to the resulting constructive and deconstructive interference. In an image detector with a finite aperture, such as a human eye, these varied patches of intensity appear as optical “speckles,” as some small portions of the image look brighter than other small portions. Further, this spot-to-spot intensity difference can vary depending on observer's position, which makes the speckles appear to change when the observer moves.
Turning now to FIG. 1, illustrated therein is a prior art system 100 in which an observer 102 may perceive speckle. Specifically, a coherent light source 101, which for discussion purposes will be a laser, delivers a coherent beam 104 to a modulation device 103. The modulation device 103 modulates the coherent beam 104 into a modulated coherent beam 105 to form an image on a projection medium, such as the projection screen 107 shown in FIG. 1.
As the projection screen 107 surface has a random roughness, i.e., as it includes tiny bumps and crevices that are randomly distributed, the reflected light 108 has portions that combine and portions that cancel. As a result, the observer 102 views an image 106 that appears to be speckled. The presence of speckle often tends to perceptibly degrade the quality of the image produced using the laser projection system.
There is thus a need for an improved speckle-reducing system for use with image projection systems.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.